Beyond distance and direction: the brain represents target locations non-metrically.

In their day-to-day activities human beings are constantly generating behavior, such as pointing, grasping or verbal reports, on the basis of visible target locations. The question arises how the brain represents target locations. One possibility is that the brain represents them metrically, i.e. in terms of distance and direction. Another equally plausible possibility is that the brain represents locations non-metrically, using for example ordered geometry or topology. Here we report two experiments that were designed to test if the brain represents locations metrically or non-metrically. We measured accuracy and variability of visually guided reach-to-point movements (Experiment 1) and probe-stimulus adjustments (Experiment 2). The specific procedure of informing subjects about the relevant response on each trial enabled us to dissociate the use of non-metric target location from the use of metric distance and direction in head/eye-centered, hand-centered and externally defined (allocentric) coordinates. The behavioral data show that subjects' responses are least variable when they can direct their response at a visible target location, the only condition that permitted the use of non-metric information about target location in our experiments. Data from Experiments 1 and 2 correspond well quantitatively. Response variability in non-metric conditions cannot be predicted based on response variability in metric conditions. We conclude that the brain uses non-metric geometrical structure to represent locations.